LATERAL APPENDAGES OF CRUSTACEA. 



235 



It is sometimes divided into several distinct rings, as in the 

 Scillce ; but in general there is no separation, and it is formed 

 of a single piece, which seems to represent seven segments joined 

 together. Sometimes it is moveable, and distinct from the 

 thorax (Fig. 458) ; sometimes, on the contrary, it is joined to 

 this second part of the body, which in its turn is composed of 

 distinct rings, articulating with each other in some species, but 

 in others united into a single mass. 



773. Of the antennae, there are nearly always two pair; 

 and they are generally thread-like, and very much elongated 

 (Fig. 459, as and ai). The legs originate by pairs from the 

 different thoracic rings ; and they often amount to seven pairs, 

 as for example in the "Woodlouse (Fig. 457), and Sand-hopper 

 (Fig. 466) ; but in other instances, as we see in Crabs (Fig. 458) 

 and Cray-fish (Fig. 471), their number is reduced to five pairs ; 

 those appendages, which in the former constituted the four 

 anterior legs, being subjected in the latter to other uses, and 

 transformed into organs of mastication ( 775). There are also 

 very great differences in their structure : in some Crustacea 

 they are leaf-like, membranous, and fitted for swimming only 

 (Fig. 490) ; in others they look like little columns jointed 

 together, and adapted for walking only ; 

 in others, while yet remaining appro- 

 priate to this last kind of locomotion, 

 they are destined to be used also as so 

 many little spades for digging in the 

 earth, and then they are enlarged and 

 laminated at the end (Fig. 460) ; 

 lastly, in others, they end in pincers, 

 and thus become instruments of 

 prehension, at the same time that 

 they fulfil their ordinary functions in 

 locomotion (Figs. 471 and 505). In 

 swimming Crustacea, such as Cray- 

 fish, Lobsters, Prawns, &c. (Fig. 459), 

 the abdomen generally attains a con- 

 siderable development, and ends in a 

 large fin, in such a manner as to become the principal agent in 



FJO. 460. HIPPA. 



